System and methods for quantum key distribution over WDM links

ABSTRACT

A system and a method for quantum key distribution between a transmitter and a receiver over wavelength division multiplexing (WDM) link are disclosed. The method includes providing one or more quantum channels and one or more conventional channels over the WDM link; assigning a different wavelength to each of the one or more quantum channels and each of the one or more conventional channels; transmitting single photon signals on each of the one or more quantum channels; and transmitting data on each of the one or more conventional channels. The data comprises either conventional data or trigger signals for synchronizing the transmission of the single photon signals on the quantum channels. All channels have wavelengths around 1550 nm. The WDM link can be a 3-channel WDM link comprising two quantum channels for transmitting single photon signals and one conventional channel for transmitting conventional data or triggering signals.

FIELD OF THE INVENTION

The present invention relates to a communication system and a method forcommunicating encrypted data. In particular, the present inventionrelates to the technique known as quantum key distribution overwavelength division multiplexing (WDM) links.

TECHNICAL BACKGROUND OF THE INVENTION

The purpose of cryptography is to exchange messages in perfect privacybetween a transmitter and a receiver by using a secret random bitsequence known as a key. Once the key is established, subsequentmessages can be transmitted safely over a conventional channel. For thisreason, secure key distribution is a fundamental issue in cryptography.Unfortunately, the conventional cryptography provides no tools toguarantee the security of the key distribution because, in principle,classical signals can be monitored passively. The transmitter andreceiver have no idea when the eavesdropping has taken place.

However, secure key distribution is possibly realized by using thetechnology of quantum key distribution (QKD). Quantum key distributionis believed to be a natural candidate to substitute conventional keydistribution because it can provide ultimate security by the uncertaintyprinciple of quantum mechanics, namely, any eavesdropping activitiesmade by an eavesdropper will inevitably modify the quantum state of thissystem. Therefore, although an eavesdropper can get information out of aquantum channel by a measurement, the transmitter and the receiver willdetect the eavesdropping and hence can change the key.

A variety of systems for carrying out QKD over an optical fiber systemhave been developed. Quantum cryptography has already been applied tothe point-to-point distribution of quantum keys between two users. Asshown in FIG. 1, quantum cryptography system in the prior art employstwo distinct links. Of them, one is used for transmission of a quantumkey by an optical fiber, while the other carries all data by internet oranother optical fiber.

However, it is desirable to apply quantum cryptography in currentlydeployed commercial optical network. Yet only several studies on quantumkey distribution over 1,300 nm network have been reported to date. Oneproblem of the reported system is that it is difficult to transmitsignals over a long distance at 1,300 nm in standard single mode fibers.Thus, quantum key distribution with wavelengths around 1,550 nm over thelong distance is preferred. In addition, it is considered that no strongsignals (e.g. conventional data) should exist in network with quantumchannels or that a large spacing of wavelengths between a quantumchannel and a conventional channel is needed to lower the interferencefrom the strong signal.

However, this is not true in the installed commercial optical networkbecause there are many strong signals that can cause severe interferenceto the quantum channel in the current optical fiber communicationsnetwork employing WDM transmission.

SUMMARY OF THE INVENTION

Generally, the present invention provides a communication system forquantum key distribution in which the quantum key distribution can beimplemented in current commercial optical links by simply adding awavelength for a quantum channel as quantum key distribution.

The present invention provides a method of quantum key distributionbetween a plurality of transmitting units and a plurality of receivingunits over a wavelength division multiplexing (WDM) link, whichcomprises: 1) providing a plurality of WDM channels over the WDM linkfor coupling the transmitting units and the receiver units,respectively, the WDM channels comprising a plurality of quantumchannels and a plurality of conventional channels; 2) assigning adifferent wavelength to each of the WDM channels; 3) transmitting singlephoton signals on each of the quantum channels; and 4) transmitting dataon each of the conventional channels, the data comprising eitherconventional data or trigger signals for synchronizing the transmissionof the single photon signals on the quantum channels.

In preferred embodiments of the invention, the wavelengths assigned tothe WDM channels are at around 1,550 nm.

The present invention further provides a communication system forquantum key distribution at wavelengths around 1,550 nm over awavelength division multiplexing (WDM) optical link, which comprises aplurality of transmitting units comprising a plurality of quantumtransmitting units and a plurality of conventional transmitting units; aplurality of receiving units comprising a plurality of quantum receivingunits and a plurality of conventional receiving units; and a WDM linklinking the transmitting units to the receiving units. Moreover, the WDMlink comprises a plurality of WDM channels, and the WDM channels mayfurther comprise a plurality of quantum channels for communicatingsingle photon signals between the quantum transmitting units and thequantum receiving units, respectively; and a plurality of conventionalchannels for communicating data between the conventional transmittingunits and the conventional receiving units, respectively.

In some embodiments of the invention, the data transmitted on theconventional channels comprises either conventional data or triggersignals for synchronizing the transmission of the single photon signalson the quantum channels. Furthermore, each of the WDM channels isassigned a wavelength different from others so that the WDM channels aremultiplexed in wavelengths over the WDM link.

According to an aspect of the present invention, it is possible torealize quantum key distribution between specific users (e.g. between atransmitter and a receiver) over a WDM link by using WDM technology. Thetransmitter may comprise one or more quantum transmitting units and oneor more conventional transmitting units, the receiver may comprise oneor more quantum receiving units corresponding to the one or more quantumtransmitting units, respectively, and one or more conventional receivingunits corresponding to the one or more conventional transmitting units,respectively. Moreover, the WDM link linking the transmitter and thereceiver may comprise one or more quantum channels for communicatingsingle photon signals between the one or more quantum transmitting unitsand the one or more quantum receiving units, respectively, and one ormore conventional channels for communicating data between the one ormore conventional transmitting units and the one or more conventionalreceiving units, and the data comprising either conventional data ortrigger signals for synchronizing the transmission of the single photonsignals on the quantum channels. Furthermore, each of the conventionalchannels and the quantum channels may be assigned a wavelength differentfrom others so that the conventional channels and the quantum channelscan be multiplexed in wavelengths over the WDM link.

According to another aspect of the present invention, the WDM link ofthe communication system may be a 3-channel WDM link, which comprisestwo quantum channels and a conventional channel. The data transmittedover the conventional channel may include trigger signals forsynchronizing the quantum channels. Thus, the conventional channel canalso serve as a trigger channel to synchronize the system. Each of theconventional channels and the quantum channels is assigned a wavelengthdifferent from others, and the conventional channel and the quantumchannels are multiplexed by wavelength at around 1,550 nm over the WDMlink, which is suitable for long-haul transmission.

Based on the WDM technology which combines many different wavelengthsinto a single optical fiber provided by the WDM link, the quantum keydistribution is easily conducted in the current commercial fiber linksby sharing a common fiber with conventional communication signals.

Moreover, a differential phase modulation technology is employed in thepresent invention to overcome an influence of temperature shifts andphase shifts on the system, which also makes the system stable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a communication system for quantum key distribution in theprior art.

FIG. 2 shows a schematic view of a communication system for quantum keydistribution over a multi-user WDM network according to the presentinvention.

FIG. 3 shows a schematic view of quantum key distribution over a WDMlink according to the present invention.

FIG. 4 shows a schematic view of an embodiment of quantum keydistribution over a 3-channel WDM link according to the presentinvention.

FIG. 5 shows an auto-compensation structure using a differential phasemodulation technology employed in a quantum channel of the presentinvention.

FIGS. 6 a and 6 b show a detailed structure of the quantum keydistribution over the 3-channel WDM link as shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail with reference to thedrawings.

WDM is the key technology adopted in the present invention, which makesuse of the parallel property of light to combine many differentwavelengths into a single optical fiber. Thus it is possible to fulfillquantum key distribution over multi-user WDM network according to thepresent invention. By virtue of WDM, the system can establishsimultaneously as many distinct secret keys as allowed by the number ofwavelengths supported by the WDM network.

For example, a communication system for quantum key distribution overmulti-user WDM network according to one embodiment of the presentinvention is shown in FIG. 2. The communication system includes Nquantum channels assigned with wavelengths from λ₁ to λ_(N) for linkingN quantum transmitting units 130 and N quantum receiving units 140 overa WDM link, and M conventional channels assigned with wavelengths fromλ_(N+1) to λ_(N+M) for linking M conventional transmitting units 330 andM conventional receiving units 340 over the WDM link (where N and M arepositive integers). The WDM link comprises array waveguide gratings(AWG) 402 and 401 and a single optical fiber 500. In the embodiment, thequantum channels and the conventional channels with distinct wavelengths(from λ₁ to λ_(N+M)) are multiplexed into the single optical fiber 500by using the AWG 401 and the AWG 402. Thus, it is possible to realizequantum key distribution between specific quantum transmitting units andquantum receiving units by using WDM technology.

FIG. 3 shows an embodiment of quantum key distribution between specificusers (e.g. between a transmitter and an intended receiver) among aplurality of users over a WDM link according to the present invention.As shown in the FIG. 3, the transmitter 711 has one or more quantumtransmitting units and one or more conventional transmitting units, andthe receiver 721 has one or more quantum receiving units, each of whichcorresponds to one of the one or more quantum transmitting units,respectively, and one or more conventional receiving units, each ofwhich corresponds to one of the one or more conventional transmittingunits.

The WDM link, linking the transmitter 711 and the receiver 721,comprises an AWG 401, an optical fiber 501 and another AWG 402. The WDMlink is provided for multiplexing one or more quantum channels betweenthe quantum transmitting units and the corresponding quantum receivingunits for communicating single photon signals, and one or moreconventional channels between the conventional transmitting units andthe conventional receiving units for communicating data. In theembodiment, the data further includes trigger signals for synchronizingthe transmission of the single photon signals on the quantum channels.Moreover, each of the conventional channels and the quantum channels isassigned a wavelength different from others, and the conventionalchannels and the quantum channels are multiplexed by wavelengths ataround 1,550 nm over the WDM link.

FIG. 4 shows an embodiment of quantum key distribution between atransmitter and a receiver over a 3-channel WDM link. The transmitter712 comprises a first quantum transmitting unit 110, a second quantumtransmitting unit 210, and a conventional transmitting unit 310. Thereceiver 722 includes a first quantum receiving unit 120 correspondingto the first quantum transmitting unit 110, a second quantum receivingunit 220 corresponding to the second quantum transmitting unit 210, anda conventional receiving unit 320 corresponding to the conventionaltransmitting unit 310. The 3-channel WDM link comprises an AWG 401, anoptical fiber 502 and another AWG 402, for multiplexing two quantumchannels 100 and 200 and one conventional channel 300. The quantumchannels 100 and 200 is provided between the two quantum transmittingunits 110 and 210 and the two quantum receiving units 120 and 220 fortransmitting single photon signals (quantum keys), respectively. Theconventional channel 300 is provided between the conventionaltransmitting unit 310 and the conventional receiving unit 320 fortransmitting data. In the embodiment, the data includes trigger signalsS1 which is transmitted to the quantum transmitting units 110 and 210,and trigger signals S2 which is transmitted to the quantum receivingunits 120 and 220, so as to synchronize the transmission of the singlephoton signals on the quantum channels 100 and 200. The two quantumchannels 100 and 200 and the conventional channel 300 are assigned withdifferent wavelengths, λ₁, λ₂ and λ₃, respectively, at around 1,550 nmwhich is compatible with the standard optical links.

In this manner, quantum key distribution can be conveniently implementedin the current commercial optical links by simply adding anotherwavelength thereto for the quantum channel as quantum key distribution.Furthermore, at the optical wavelength of 1,550 nm, the fiber losses are0.2 dB/km, which translates into a large increase in transmissiondistance when compared with that at 1,300 nm at the same bit rate for aquantum cryptographic system.

The BB84 protocol can be employed in the quantum channels 100 and 200.In order to implement BB84 protocol, there must be four states in twonon-orthogonal bases, each of which has two orthogonal states. Forexample, the four phases {0, π/2, π or 3π/2} can play the role of thefour states. Moreover, {0, π} corresponds to one basis that can berealized via choosing measurement basis phase shift 0. Similarly, {π/2,3π/2} is the other basis that corresponds to measurement choice of phaseshift π/2. The four states can be expressed in the following,For “0”, |‘0’

=1/√{square root over (2)}(|0

+|π/2

)For “1”, |‘1’

=1/√{square root over (2)}(|π

+|3π/2

)

From the wave functions, it is obvious that there is equal probabilityof 50% for phase shift 0 and π/2, respectively, for logic 0. So is logic1.

An auto-compensation structure using a differential phase modulationtechnology is employed in the quantum channels of the present invention.As shown in FIG. 5, for example, in a quantum channel 100 (which issimilar to a quantum channel 200), at the transmitter 712, a phaseshift, ΔA, provided by a phase modulator 112, is added to a first pulsein two neighboring pulses both of which travel from the receiver 722 tothe transmitter 712. Another phase shift, ΔB, provided by a phasemodulator 122, at the receiver 722 will be also added to a second pulsewhen both the pulses return to the receiver side after being reflectedby a Faraday rotating mirror 111. When the first pulse and the secondpulse delayed by a delay means 127 arrive at a beam splitter 123,interference will happen, and the phase difference will be ΔA−ΔB.Therefore, only the phase difference has been retained. This arrangementenables the structure to compensate errors caused by temperature shifts,polarization changes and path variations experienced by the two pulsestraveling in the interference section, because each of the two pulses,which will interfere at the receiver side of each quantum channel,experiences the same variations while traveling the same distance. Herewe assume that another phase shift, δ, caused by the temperature shift,polarization variation and distance variations, is put on both of thepulses in the same channel.

The phase shift, δ, often changes at a different time for the variationby the factors mentioned above. However, it is nearly equal for the twoneighboring pulses because they experience similar changes in thechannel as those factors mentioned above vary relatively slowly withinthe time separation between the two neighboring pulses. For the firstpulse, it has a phase shift, ΔA+δ, but there is a phase shift, δ+ΔB, forthe second pulse. Hence, in the interfering section at the receiverside, the phase difference between the two returning pulses is ΔA−ΔBbecause the phase shift, δ, caused by the factors mentioned above willhave been cancelled. Since the quantum channel 200 is similar to thequantum channel 100, the scheme of the quantum channels 100 and 200 ofthe present invention can overcome fluctuations caused by temperature,polarization and distance variations. Theoretically, it can obtainperfect interference in the scheme.

A detailed structure and principles of the quantum key distribution overa 3-channel WDM link are described with reference to FIGS. 6 a and 6 b.

In the quantum channel 100, at the receiver 722, a laser 124 launches apulse string with power of 0 dBm into the WDM link via a circulator 125.Each pulse in the pulse string will be split into two pulses through a50/50 beam splitter 123, a first pulse and a second pulse. The firstpulse passes through an upper path 1231 with a delay of 26 ns set by adelay means 127 (e.g. a delay line of an optical fiber) before hitting apolarization beam splitter 121. A phase modulator 122 in the upper path1231 is not used until a second pulse returns from the transmitter. Asecond pulse passes through a lower (shorter) path 1232 directly to theinput port of the splitter 121.

After passing through the splitter 121, the two pulses with orthogonalpolarizations and a delay of 26 ns between them are obtained. These twopulses then enter into an array waveguide grating (AWG) 402, propagatethrough a single-mode fiber 502 of e.g. 8.5 km, enter into another arraywaveguide grating (AWG) 401, and then exit from the AWG 401 in channel100 at the transmitter 712.

The pulses are again split by a 90/10 beam splitter 115, and the photonscoming out from the 90 percent port of the splitter 115 are detected bya detector 113 for controlling a variable attenuator 114 to attenuatethe returning pulses to obtain single-photon pulses. The two pulsescoming out of the 10% port of the splitter 115 will pass through theattenuator 114 first without attenuation. They will then arrive at aFaraday mirror 111 through a phase modulator 112. The polarizations ofthe two pulses are rotated by 90° after they are reflected by theFaraday rotating mirror 111.

A random phase shift of 0, π/2, π or 3π/2 generated by a random datasignal generator (not shown) is then inserted into the first of the tworeturn pulses by the phase modulator 112. The two return pulses are nextattenuated to yield a single photon within a pulse when they passthrough the attenuator 114 again. A trigger signal S1 generated from adetector 313 is used to synchronize with the phase modulator 112 tomodulate the first return pulse from the Faraday mirror 111 and with anattenuation control signal from the detector 113 to attenuate bothreturn pulses into single photons. Here the trigger signal S1 from thedetector 313 should have an appropriate delay to synchronize the phaseshift single from the data signal generator with the first return pulse.Also, the signal from detector 113 used to control attenuator 114 has anelectrical delay in order to attenuate both light pulses when they passthrough it in their return trip. Finally the two pulses return to thereceiver 722 via opposite paths between the polarization beam splitter121 and the beam splitter 123 after passing through the AWG 401, the 8.5km standard single mode fiber 502 and the AWG 402. Hence, they canarrive at the beam splitter 123 at the same time and generateconstructive or destructive interference at the beam splitter 123 toenable single photons to be detected by a single-photon detector 126.

The receiver 722 can randomly and independently select a measurementbasis through setting a phase shift of 0 or π/2 in the phase modulator122, which is synchronized by a trigger signal S2 derived from the pulsereturning from the mirror 311 in the conventional data channel 300. Theoutcomes are stored in a computer 600. All fibers on receiver's side arepolarization-maintaining fibers, which is necessary for the system toguarantee the polarizations of the two single photon pulses that willinterfere are invariant after passing through the different paths of theinterferometer.

A second quantum channel 200, similar to the quantum channel 100,comprises a Faraday mirror 211, a phase modulator 212, a detector 213, avariable attenuator 214, a 90/10 beam splitter 215, an AWG 401, a fiber500, an AWG 402, a polarization beam splitter 221, a phase modulator222, a beam splitter 223, a laser 224, a circulator 225, a single photondetector 226 and a delay means 227. For the reason that theconfiguration and principles of channel 200 is similar to the quantumchannel 100, except that a time delay set by the delay means 227 of thequantum channel 200 is 21 ns and an independent measurement basis andrandom phase shifts that are independent of channel 100, the detaileddescription of the quantum channel 200 is omitted.

In the conventional channel 300, a common laser 324 emits a pulse withthe power of 2 dBm into a 50/50 beam splitter 321, on receiver's side.The pulse then enters AWG 402 after passing the 50/50 beam splitter 321,travels in the 8.5 km single-mode fiber 500 and then through AWG 401,after which one-half of the pulse will be detected by a detector 313.The detected signal is used as a first trigger signal S1 to synchronizethe phase modulators 112 and 212 with their respective pulses in quantumchannels 100 and 200 through appropriate delays. The other half of thepulse will be reflected by a mirror 311 to return to the receiver, andwill be detected by a detector 326 to generate a second trigger signalS2 to trigger the single photon detectors 126 and 226 to measure theinterference of the quantum signals and to trigger the phase modulators122 and 222 to select a measurement basis on the receiver's side,respectively.

The data communication channel 300 may also function as a regularoptical communication channel which has high laser powers, e.g., 2 dBmemitted by the laser 324 in this embodiment. The wavelengths and thepulse widths of the three channels are listed in Table 1.

TABLE 1 wavelength and pulse width Channel 100 200 300 Wavelength (nm)1549.33 1551.18 1557.35 Pulse width (ns) 2.5 2.5 20

BB84 protocol has been implemented in this system. We use 100 kHzsignals for phase modulation and synchronization. The pulse widths are2.5 ns for quantum channels 100 and 200, and 20 ns for conventionalchannel 300, shown in Table 1. In order to reduce the crosstalk amongthe channels, especially between weak quantum channels 100 and 200, andthe strong signal channel (the conventional channel 300), thewavelengths have to be arranged carefully. Here the spacing betweenquantum channel 100 and conventional channel 300 is about 8 nm, and thatbetween quantum channel 200 and conventional channel 300 is about 6 nm.

The single photon detectors 126 and 226 are employed in the embodimentto measure the single photons. The dark count of the single photondetectors 126 and 226 is 40 Hz in the gated mode of 100 kHz with ameasurement width of 2.5 ns, so the probability of measuring the darkcount is 4.0×10⁻⁴. The efficiency of the single photon detectors 126 and226 is more than 10%. On the transmitter's side, the average photoncount per pulse should be less than 0.1 in order to guarantee that asingle photon is obtained in each pulse in the embodiment when the pulsepasses through the variable attenuator 114 again. For an overalltransmission loss of 17 dB, about 2% of single photons can be detected.On considering the 3 dB loss due to BB84 protocol, about 1% of singlephotons can be used for quantum key distribution theoretically.

TABLE 2 Experimental Results Channel 100 200 Key rate (kb/s) 0.75 0.49Error probability (%) 2.2 4.396

Experimentally, the count rate of the single photon detector 126 and 226is 100 k counts/s and its efficiency is above 10%. In order to guaranteesingle photon in a pulse, the average photon count per pulse should bebelow 0.1 in the embodiment of the present invention. Therefore, countrate should be below 10 k/s at variable attenuators 114 and 214.According to the embodiment, the experimental count rate obtained is7.67 k/s. After considering the transmission efficiency, error rate anddetector efficiency, a 0.75 kbps quantum key has been obtained inchannel 100, where the crosstalk causes an error probability of 2.2percent, mostly derived from channel 300 and much less from channel 200because the single photon signal in channel 200 is very weak. Similarly,in channel 200, the quantum key rate is 0.49 kbps and the crosstalk alsocauses an error probability of 4.396%. The crosstalk in channel 200 islarger than that in channel 100 because its wavelength is closer to thatof the conventional communication channel than is the wavelength ofchannel 100.

While this invention has been described in conjunction with a fewembodiments thereof, it will be understood for those skilled in the artto put this invention into practice in various other manners. It isappreciated that the scope of the invention is defined by the appendedclaims and should not be restricted by the description discussed in thesummary and/or the detailed description of the preferred embodiments.

1. A method of quantum key distribution, comprising: transmitting from areceiver via a conventional channel a light pulse for generating a firsttrigger signal for triggering a plurality of modulators at atransmitter; receiving at a plurality of quantum receivers a pluralityof modulated single photon signals via a plurality of quantum channels,wherein the plurality of modulated single photon signals are indicativeof quantum key information imposed by the plurality of modulators andwherein the conventional channel and the plurality of quantum channelsare multiplexed in wavelengths; and receiving at the receiver via theconventional channel a reflected portion of the light pulse forgenerating a second trigger signal for triggering a plurality ofmodulators within the plurality of quantum receivers.
 2. The methodaccording to claim 1, wherein the plurality of quantum channels and theconventional channel are multiplexed in a WDM link.
 3. The methodaccording to claim 2, wherein the wavelengths assigned to the pluralityof quantum channels and the conventional channel are at around 1,550 nm.4. The method according to claim 3, wherein the wavelengths range from1,475 nm to 1,590 nm.
 5. The method according to claim 4, wherein thewavelength assigned to one of the plurality of quantum channels is at1,549.33 nm or 1,551.18 nm, and the wavelength assigned to theconventional channel is at 1,557.35 nm.
 6. The method according to claim1, further comprising: generating an interference signal in response toat least one of the plurality of modulated single photon signals; andextracting the quantum key information by processing the interferencesignal.
 7. The method according to claim 1, further comprising:initiating a first light pulse and a second light pulse; andtransmitting via at least one of the plurality of quantum channels thefirst light pulse and the second light pulse for use by the transmitterfor generating at least one of the plurality of modulated single photonsignals.
 8. The method according to claim 1, wherein at least one of theplurality of modulators within the plurality of quantum receivers is adifferential phase modulator.
 9. The method according to claim 1,wherein at least one of the plurality of quantum channels uses a BB84protocol.
 10. A communication system for quantum key distribution,comprising: a receiving unit, configured for: (i) transmitting via aconventional channel a light pulse for generating a first trigger signalby a transmitter for triggering a plurality of transmitter modulators atthe transmitter and for (ii) receiving via the conventional channel areflected version of the light pulse for generating a second triggersignal for synchronizing a plurality of receiver modulators; and aquantum receiving unit, comprising the plurality of receiver modulators,configured for receiving a plurality of modulated single photon signalsvia a plurality of quantum channels, wherein the plurality of modulatedsingle photon signals are indicative of quantum key information andwherein the conventional channel and the plurality of quantum channelsare multiplexed in wavelengths.
 11. The communication system accordingto claim 10, further comprising an array waveguide grating fordemultiplexing the plurality of quantum channels and the conventionalchannel.
 12. The communication system according to claim 11, wherein atleast one of the plurality of modulated single photon signalstransmitted on the plurality of quantum channels is generated from alaser at a wavelength ranging from 1,475 nm to 1,590 nm.
 13. Thecommunication system according to claim 10, wherein the plurality ofquantum channels and the conventional channel are multiplexed in a WDMlink.
 14. The communication system according to claim 11, wherein eachof the plurality of receiver modulators applies a differentialmodulation to one of the plurality of the modulated single photonsignals and wherein each of the plurality of receiver modulatorscomprises a short arm and a long arm.
 15. The communication systemaccording to claim 10, wherein the quantum receiving unit initiates afirst light pulse and a second light pulse and transmits, via at leastone of the plurality of quantum channels, the first light pulse and thesecond light pulse for use by the transmitter for generating at leastone of the modulated single photon signals.
 16. The communication systemaccording to claim 10, wherein the receiving unit further includes alaser source for generating the light pulse.
 17. The communicationsystem according to claim 16, wherein the conventional receiving unitcomprises a detector configured for detecting the reflected version ofthe light pulse and for generating the second trigger signal.
 18. Amethod for quantum key distribution comprising: transmitting, via aplurality of quantum channels, a plurality of modulated single photonsignals, wherein each of the plurality of modulated single photonsignals is indicative of quantum key information; receiving, via aconventional channel, a light pulse for generating a first triggersignal for triggering the modulation of the plurality of modulatedsingle photon signals, wherein the plurality of quantum channels and theconventional channel are multiplexed in wavelength; and transmitting,via the conventional channel, a reflected portion of the light pulse foruse by a receiver for generating a second trigger signal.
 19. The methodof claim 18, further comprising: receiving a plurality of light pulsesvia the plurality of quantum channels; modulating a reflected portion ofthe plurality of light pulses received via the plurality of quantumchannels in response to the first trigger signal; and generating theplurality of modulated single photon signals by attenuating themodulated plurality of light pulses.
 20. A communication system forquantum key distribution comprising: a quantum transmitting unitconfigured for transmitting, via a plurality of quantum channels, aplurality of modulated single photon signals, wherein the plurality ofmodulated single photon signals is indicative of quantum keyinformation; and a transmitting unit configured for: receiving, via aconventional channel, a light pulse and responsively generating a firsttrigger signal for triggering the modulation of the plurality ofmodulated single photon signals, wherein the quantum channel and theconventional channel are multiplexed in wavelength and for transmitting,via the conventional channel, a reflected portion of the light pulse foruse by a receiver for generating a second trigger signal.
 21. Thecommunication system of claim 20, wherein the transmitting unit furthercomprises: a detector for detecting a portion of the light pulse andresponsively generating the first trigger signal; and a Faraday mirrorfor generating the reflected portion of the light pulse.
 22. Thecommunication system of claim 20, wherein the quantum transmitting unitreceives via the plurality of quantum channels a plurality of lightpulses, and wherein the quantum transmitting unit further comprisesphase modulators for modulating a portion of the plurality of lightpulses in response to the first trigger signal and variable attenuatorsfor attenuating the modulated portion of the plurality of light pulsesto generate the plurality of modulated single photon signals.
 23. Thecommunication system of claim 20, further comprising an array waveguidegrating for multiplexing the plurality of quantum channels and theconventional channel.